Optical amplifiers are widely used in optical networks to extend the reach of optical systems by compensating for line and device losses. A signal passing through the active medium of an optical amplifier is amplified by a channel gain factor dependant both on the pump laser power and the total number of channels that carry a signal passing through the amplifier. The number of channels that carry a signal to be amplified in the network may change due to network upgrades, dynamic reconfigurations, and channel failures.
Currently, automatic gain control (AGC) compensates for variations in amplifier channel gain due to changes in the number of channels that carry a signal by monitoring the total input and total output power of the amplifier and adjusting pump laser power to maintain a constant average gain. However, because only total input and output powers are monitored, as opposed to the powers of individual channels, the gain of individual channels cannot be monitored or controlled.
An article by W. J. Tomlinson entitled “Dynamic Gain Equalization for Next-Generation DWDM Transport Systems” published in the Digest of LEOS Topical Meetings, Paper WD 1.1 (2001), describes the use of high-resolution dynamic gain equalizers (DGEs) for dynamic gain control on a per-channel basis. However, since DGEs are capable only of attenuating and not amplifying, they can only compensate for excess gain and not for insufficient gain. Additionally, the use of DGEs in metropolitan and regional area networks is unwelcome because of the associated capital and operational costs. Finally, the increased losses introduced by DGEs may necessitate additional optical amplifiers, which increases costs and degrades optical signal to noise ratio (OSNR).
Therefore, there is a need in industry for the development of alternative solutions for monitoring and controlling the gain of an amplifier in an optical system, which would include monitoring of individual channels and would be simple and cost-efficient.